I use Facebook on my computer. I have this problem when I send someone any message, and the message gets seen, the time of seen is the same as the time I sent it.
This can happen for several reasons. The most likely is that the recipient has Facebook open on one of their devices, causing any message to be shown as read as soon as it arrives (aka the same time it was sent). Alternatively, the recipient could have a message client reading their Facebook messages for them , thus causing every message to be read as soon as it arrives.